Sarit Kumar Das

Present study depicts the transient response of multi-pass plate heat exchangers (PHEs) considering flow maldistribution from port to channels. Apart from the flow maldistribution, fluid axial dispersion is also considered to take care of the fluid backmixing and other deviations from plug flow. It is assumed that each multi-pass PHE is a combination of single-pass PHEs and in each heat exchanger the fluid is distributed non-uniformly amongst channels. The fluid velocity varies from channel to channel within each module of the heat exchanger so also the heat transfer coefficient. The solution techniques have been presented here for 1-2 pass and 2-2 pass arrangements. The first one is solved by analysing successive modules of the heat exchanger and the next one by iterating the responses between two heat exchanger modules. The solution for each module has been obtained analytically by using Laplace transform followed by numerical inversion from frequency domain. The results show the effect of flow maldistribution and its effect combined with the conventional heat exchanger parameters in the transient regime. It is observed that the transient characteristics such as response delay, asymptotic value and time constant are strongly dependent on the multi-pass flow arrangement, maldistribution and backmixing characterised by axial dispersion. © 2007 Elsevier B.V. All rights reserved.

The flow of fuel and oxidant through a PEMFC is analyzed for prediction of maldistribution. Flow distribution of both fuel and oxidant from the port to the individual cells critically control the performance of a PEMFC stack in combination. The distribution of fluids was simulated by analytical approach utilizing flow channeling model of a manifold. A detailed numerical modeling is also carried out considering flow in each cell between the electrodes as flow through an equivalent porous medium offering identical resistance. The results show a close match between the analytical and numerical results. The parametric study reveals that flow rate and port size plays major role determining maldistribution of the fluids, which can be considerably skewed when large numbers of cells are stacked for larger power output.

Offset strip fins are used in Compact Heat Exchangers. A new technique for the prediction of heat transfer characteristics of these surfaces has been established. The method utilizes an analogy between heat and momentum transfer proposed for short thermally developing laminar flow. The present work exploits the intuition that the analogy can be utilized for any repeated flow structure with heat transfer from surfaces that are also repetitive in array. In the offset strip fin geometry, the original form of this analogy shows a constant bias, which depends on the fin geometry. This leads to the incorporation of the fin geometrical parameters to a new analogy equation resulting in a generalized form of equation for the entire range of offset strip fins. The results clearly indicate that the heat transfer characteristics of offset strip fin surfaces could be predicted by measuring pressure drop across them alone. © 2006 R.T. Edwards, Inc.

Introduction to nanofluids--their properties, synthesis, characterization, and applications Nanofluids are attracting a great deal of interest with their enormous potential to provide enhanced performance properties, particularly with respect to heat transfer. In response, this text takes you on a complete journey into the science and technology of nanofluids. The authors cover both the chemical and physical methods for synthesizing nanofluids, explaining the techniques for creating a stable suspension of nanoparticles. You get an overview of the existing models and experimental techniques used in studying nanofluids, alongside discussions of the challenges and problems associated with some of these models. Next, the authors set forth and explain the heat transfer applications of nanofluids, including microelectronics, fuel cells, and hybrid-powered engines. You also get an introduction to possible future applications in large-scale cooling and biomedicine. This book is the work of leading pioneers in the field, one of whom holds the first U.S. patent for nanofluids. They have combined their own first-hand knowledge with a thorough review of theliterature. Among the key topics are: Synthesis of nanofluids, including dispersion techniques and characterization methods Thermal conductivity and thermo-physical properties Theoretical models and experimental techniques Heat transfer applications in microelectronics, fuel cells, and vehicle engines This text is written for researchers in any branch of science and technology, without any prerequisite.It therefore includes some basic information describing conduction, convection, and boiling of nanofluids for those readers who may not have adequate background in these areas. Regardless of your background, you'll learn to develop nanofluids not only as coolants, but also for a host ofnew applications on the horizon. © 2008 John Wiley & Sons, Inc.

Experiments were conducted to study the effect of tube inclination on nucleate pool boiling heat transfer for different tube diameters and surface roughness values. The results show that as the tube is tilted from the vertical to the horizontal, the temperatures at the top and bottom (with respect to circumference) increase and decrease, respectively. The increase and decrease is such that they almost compensate for each other, resulting in very little variation of the average heat transfer coefficient with tube inclination. The increase in bubble sliding length at the bottom wall and decrease in bubble sliding length at the top wall are thought to be the reasons for this behaviour. Experiments have been conducted with water, ethanol and acetone at atmospheric pressure, to confirm similar effects of inclination irrespective of fluid property. © 2009 Springer-Verlag.

A detailed experimental study on flow maldistribution from port to channel of a plate heat exchanger is presented. In general, flow maldistribution brings about an increase in pressure drop across the heat exchanger. This increase is found to depend on flow rate, number of channels and port size. Experiments show that analytical predictions of pressure drop including maldistribution effect are quite accurate for practical purposes. The results indicate that under identical conditions, maldistribution is more severe in Z-type plate heat exchanger compared to U type. Experiments are also carried out under non-isothermal realistic operating conditions, which show increased flow maldistribution at elevated temperature. Finally predictions are made for industrial plate heat exchangers, which show the limitation of adding additional plates beyond a certain limit. An insight to the physical aspects of maldistribution and its possible reduction through proper design strategy is also indicated.

Compact heat exchangers have drawn considerable attention in the recent years due to unprecedented growth of information and process technology and the resulting demand of highly efficient and compact heat removal devices. Until now the performance evaluation of such heat exchangers are made by and large in a substantative way without caring for the thermodynamic potential that the heat transfer device can utilise. The 'Second Law analysis' is a technique which can remove this deficiency. The exergy analysis presented in this paper has also got the unique feature of linking heat transfer to pressure drop encountered. To carryout the analysis a novel approach is suggested here. Firstly, the pressure drop data for unknown heat exchanger surface is predicted by analysing similar data with the help of Artificial Neural Network (ANN). Subsequently the heat transfer data for the same surface has been predicted by using a rediscovered analogy known as Leveque analogy in its modified form. Finally these two fold data are combined in the 'Second Law analysis' which indicates the possibility of optimizing the design of the heat exchanger for minimum irreversibility. In the whole exercise a compact regenerator bed consisting of crossed rod bundles has been used to demonstrate the methodology developed. (C) 2000 Elsevier Science S.A. All rights reserved.

An experimental and theoretical study of the effect of flow maldistribution from port to channel on the thermal performance of single and multipass plate heat exchangers is presented. In general, flow maldistribution brings about an increase in pressure drop and decrease of the thermal performance in heat exchangers. This deterioration is found to depend on flow rate, number of channels, and port size. Experiments show that analytical predictions of pressure drop and thermal performance in presence of flow maldistribution are quite accurate for practical purposes. The results indicate that under identical conditions, maldistribution is more severe in Z-type plate heat exchanger compared to U type. Multipassing is found to reduce the maldistribution effect significantly. An insight to the physical aspects of maldistribution and its possible reduction through proper design strategy are also presented. Copyright © 2005 by ASME.

A modified mixing plane approach for steady state simulation of flow field in fully baffled biological reactor is presented and discussed. Without requiring any experimental input, this approach of dividing the vessel into suitable number of connected and disconnected zones; solving steady state equation separately in each zone and then transferring information between them, provides a computationally less intensive alternative for simulating the flow in the whole vessel. Impeller used is the standard Rushton Turbine positioned at mid-height of the reactor and simulations are carried out using standard k-ε turbulence model implemented in CFD code FLUENT. Meshing is done using tetrahedral elements such that mesh size gradually increases from the center to the periphery. Most of the previous simulation works present only a few aspects of the flow field with scant importance to the energy balance in the tank and near tip turbulence. In this work, complete model prediction for velocity field and turbulence parameters (near tip and in the bulk region) are validated by comparison with experimental data. As compared to previous simulation works, the results predicted by this "Differential circumferential averaging mixing plane approach" show a better qualitative and quantitative agreement with the published experimental data. A distribution of energy dissipation in various zones of vessel is presented. Also a qualitative picture of flow field and stagnant zones inside the reactor is presented and discussed. Comparison of flow characteristics for different number of baffles shows that for the present dimension of the vessel, five baffles gives maximum enhanced mixing. © 2006 Wiley Periodicals, Inc.